1. Field of the Invention
This invention relates to a mechanism for detecting the passing of a force threshold in a linear mechanical control channel, and which can be applied more particularly to the pitch control channel of each blade of a helicopter torque-opposing tail rotor.
2. Description of the Prior Art
The pitch articulation of each blade on the hub of an articulated-type helicopter tail rotor is made up of a bearing forming a thrust block, which can be produced by a set of deep-seated ball bearings interposed between the blade connecting sleeve and a journal which is connected to the hub body via the flapping articulation.
The tracks of these ball-bearings of the pitch articulation are subject to heavy axial loading as a result of the centrifugal force transmitted by the blade. They oscillate about a small angular working range and tend in service to become scored in this area. This leads to a potentially large increase in the torque required to make pitch adjustments, and by consequence an increase in the forces developed in the pitch control system on the downside of the servocontrol which could result in this control channel rupturing.
The customary procedure for monitoring the condition of the ball bearing sets is to periodically measure the rotational torque of the connecting sleeves with the tail rotor at a standstill. This measurement is not only awkward to take, but also inaccurate and not representative of the in-service torque since the centrifugal force exerted by the blade on the ball bearing set during this operation is zero. Moreover, this check imposes constraints since it calls for dismantling work and the use of a balance to measure the rotational torque of the sleeves. The results of this measurement are also fairly difficult to interpret.